Infant botulism is the response to the neurotoxin that is produced during multiplication of Clostridium botulinum in the intestinal tract. Results obtained so far with animal models are consistent with the belief that infants are susceptible to the toxicoinfection because their intestinal microbiota do not yet include bacteria which prevents growth of the pathogen in older individuals. An intestinal flora consisting of nine species protects mice against the botulinum infection. The organisms of this flora will be tested to identify the one (or one) which is responsible for the protection. It the organism can be established as part of the intestinal flora of infant mice, the resulting animals will be tested to see if they have become resistant. The mechanism of the anti-C. botulinum action will be studied. Culture filtrates of some type B strains are strikingly more lethal for infant mice than for the adults. The neurotoxin of a representative sample will be purified and tested to see if the high infant to adult toxicity ratio is the result of the neurotoxin acting by itself.